Lawyers, professors and election law experts told NBC that there is enough to the circumstances around the payment, made by the Trump Organization's longtime attorney Michael Cohen shortly before the 2016 presidential election, that it could have violated campaign laws.

Hassen said the key issue is whether the payment was made by Cohen in an effort to help the campaign by silencing Daniels, whose real name is Stephanie Clifford. If that is the case, Hassen noted the payment would exceed the federal limits for individual campaign donations.

Cohen confirmed that he made the payment to Daniels, saying that he transferred the funds from his home equity line.

Daniels sued Trump last week, saying that the nondisclosure agreement about their alleged affair was void because Trump hadn’t signed the document.

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Others noted the payment could be legal if Trump used his own money, given the lack of limits for contributions that candidates can give their own campaigns, but could constitute a federal crime if Trump failed to disclose it in his federal campaign filings.

Citizens for Responsibility and Ethics in Washington, a financial watchdog group, filed official complaints with the Department of Justice this week asking for an investigation into whether Trump intentionally withheld the payment from his public financial report, saying he could have broken the law if the undisclosed payment was a loan.

Democrats on the House Judiciary Committee and Women's Working Group also requested information on the payment in a letter to Cohen this week that said Trump could potentially have violated tax laws and opened himself to possible blackmail over the allegations.